Therapeutic heating has long been employed by the medical profession to treat a variety of physical ailments. Currently employed techniques routinely used to heat tissue include microwave diathermy and ultrasonic therapy. Devices now used for therapeutic heating are relatively inexpensive and therefore are widely available (there are approximately 3000 ultrasonic therapy devices sold domestically each year). One serious problem associated with both microwave diathermy and ultrasonic therapy devices is the poor control provided of exposure conditions, namely, dosage. Although the practitioner can specify the exposure time, he still does not know precisely how much actual energy is absorbed by the tissue undergoing treatment, and consequently does not know how high the tissue temperature rise will be. Thus, there is a definite need for an improved exposure system having means for measuring the total ultrasonic energy absorbed by biological tissue undergoing exposure, namely, for therapy applicator apparatus capable of deriving information usable for measuring exposure dosage.
There exists a known technique for measuring the absorption coefficient of liquids below 2.0 MHz (see Karpovich, J.: "Resonance Reverberation Method for Sound Absorption Measurements," J. Acoust. Soc. Am, 16(5), 819-823, 1954; Kurtze, G. and Tamm, K.: "Measurements of Sound Absorption in Water and in Aqueous Solution of Electrolytes," Acustica, 3, 33-48, 1953; Lawley, L. E. and Reed, R. D. C.: "A Reverberation Method for the Measurement of the Absorption of Ultrasonics in Liquids," Acustica, 5, 316-322, 1955; Moen, C. J.: "Ultrasonic Absorption in Liquids," J. Acoust. Soc. Am., 23(1), 62-70, 1951; Mulders, C. E.: "Ultrasonic Reverberation Measurements in Liquids," App. Sci. Res., B1, 149-167, 1948). For this technique, a diffuse field is first created in a reverberation tank, and then the time rate of change of acoustic pressure, or decay rate, is determined when the source of ultrasonic energy is modulated. Such a technique has never, however, been applied to the present field.